Remove ExprBuiltin

It's slower than ExprVar since it doesn't compute a static
displacement. Since we're not using the throw primop in the
implementation of <...> anymore, it's also not really needed.

src/libexpr/nixexpr.hh

@@ -272,13 +272,6 @@ struct ExprOpNot : Expr
     COMMON_METHODS
 };
 
-struct ExprBuiltin : Expr
-{
-    Symbol name;
-    ExprBuiltin(const Symbol & name) : name(name) { };
-    COMMON_METHODS
-};
-
 #define MakeBinOp(name, s) \
     struct Expr##name : Expr \
     { \